#1 - Try it. If the phenol red converts to chlorophenol red, you'll end with a purplish-blue. If not, the test should be valid
#2 - Confirm it - if it worked, check it it by adding 1 drop of thiosulfate from your K2006 to your water sample and mixing BEFORE you add phenol red. If the result with thiosulfate is the same, or just slightly higher, the first test was OK.
#3 - If #1 fails, and you show chlorophenol red, add 3 drops of thiosulfate and mix before adding phenol red. You should get a meaningful result, but it may read 1 color bar high because of the thiosulfate / chlorine reaction.
When you lower pH, it will -- if you act slowly -- also lower TA. If you lower pH, and actively aerate, the TA (really, the CA, carbonate alkalinity) will drop much more rapidly.
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